Most holograms used for authentication purposes are, like the Master Card hologram, of the embossed variety. A three-dimensional image is created when light is diffracted from a finely-grooved, reflective surface. Such a structure is typical of credit card holograms. Embossed holograms discourage counterfeiting simply because they require advanced technology for production and cannot be made inexpensively in small quantities. The effectiveness of methods which use embossed holography is swiftly declining as counterfeiters become more sophisticated.
Volume holograms are more difficult to counterfeit than are embossed holograms. They can be created either as transmission or reflection holograms. In transmission holography, the reference wave passes through the recording material and diffracts into the image beam light which travels more or less in the same direction as the reference wave. In reflection holography, the reference wave reflects from structure within the volume of the hologram and forms the object wave. A variety of diffracting mechanisms can be employed in forming a volume hologram, depending on the material and its processing. One of these materials, discussed in this disclosure, is a photopolymer material. Such a material, produced by DuPont, can be processed in situ since it requires no wet processing.
Both volume and surface relief holograms are recorded as flat, planar structures. This is usually necessary in order to guarantee that a recognizable image is reconstructed from them when they are illuminated with a collimated or point source of light. Their application to complex surfaces has been avoided. Holograms of simple shape, such as flat or perhaps slightly curved, provide a potential counterfeiter with an opportunity to create a simulation of them. Many flat holograms, for example, can be copied using simple contact copying methods.